1. Field of the Invention
The present invention relates to centrifugal compressors in general and in particular to a composite roller bearing for a variable pipe diffuser.
2. Background of the Prior Art
One of the major problems arising in the use of centrifugal vapor compressors for applications where the compressor load varies over a wide range is flow stabilization through the compressor. The compressor inlet, impeller and diffuser passages must be sized to provide for the maximum volumetric flow rate desired. When there is a low volumetric flow rate through such a compressor, the flow becomes unstable. As the volumetric flow rate is decreased from a stable range, a range of slightly unstable flow is entered. In this range, there appears to be a partial reversal of flow in the diffuser passage, creating noises and lowering the compressor efficiency. Below this range, the compressor enters what is known as surge, wherein there are periodic complete flow reversals in the diffuser passage, destroying the efficiency of the machine and endangering the integrity of the machine elements. Since a wide range of volumetric flow rates is desirable in many compressor applications, numerous modifications have been suggested to improve flow stability at low volumetric flow rates.
Many schemes have been devised to maintain high machine efficiencies over a wide operation range. In U.S. Pat. No. 4,378,194, the entire diffuser configuration is varied in response to load changes in an effort to match the machine performance with the changing load demands. Adjustable diffuser flow restrictors are also described in U.S. Pat. No. 3,362,625 which serve to regulate the flow within the diffuser in an effort to improve stability at low volumetric flow rates.
A common technique for maintaining high operating efficiency over a wide flow range in a centrifugal machine is through use of the variable width diffuser in conjunction with fixed diffuser guide vanes.
U.S. Pat. Nos. 2,996,996 and 4,378,194, issued to a common assignee, describe variable width vaned diffusers wherein the diffuser vanes are securely affixed, as by bolting to one of the diffuser walls. The vanes are adapted to pass through openings formed in the other wall thus permitting the geometry of the diffuser to be changed in response to changing load conditions.
Fixedly mounting the diffuser blades to one of the diffuser walls presents a number of problems particularly in regard to the manufacture, maintenance and operation of the machine. Little space is afforded for securing the vanes in the assembly. Any misalignment of the vanes will cause the vane to bind or rub against the opposite wall as it is repositioned. Similarly, if one or more vanes in the series has to be replaced in the assembly, the entire machine generally has to be taken apart in order to effect the replacement.
The efficiency of a compressor could be greatly enhanced by varying the outlet geometry of the diffuser. In U.S. patent application Ser. No. 08/658,801, U.S. Pat. No. 5,807,071 commonly assigned, a variable geometry pipe diffuser is disclosed. That application is hereby incorporated by reference. A variable geometry pipe diffuser (which may also be termed a split-ring pipe diffuser) splits the diffuser into a first, inner ring and a second outer ring. The inner and outer rings have complementary inlet flow channel sections formed therein. That is, each inlet flow channel section of the inner ring has a complementary inlet flow channel section formed in the outer ring. The inner ring and outer ring are rotatable respective to one another. The rings are rotated to improve efficiency for varying pressure levels between a fully open position and a partially closed position. In the partially closed position the misalignment of the exit pipes of the diffuser causes an increase in noise. Rotation of the rings past an optimum design point results in excessive noise and efficiency degradation.
The geometrical tolerances within a centrifugal compressor are small. At the same time the loads within the compressor are large and dynamic in nature. In a split ring pipe diffuser the problem of maintaining tolerances in the face of the dynamic loading becomes quite onerous. There are both axial (thrust) loads and circumferential loads on the ring pair that need to be managed. The diffuser rings must be able to rotate relative to one another and at the same time, tight control over their relative positions must be maintained in order to ensure proper alignment of the flow channels and the ultimate efficiency of the compressor. The cost of maintaining the necessary tolerances in a split ring diffuser is generally very high.
Another problem with split ring diffusers is premature part wear. Lubricants are generally not used within the gas flow regions of centrifugal compressors to preclude contamination of the gases. The dynamic loads imposed upon the split ring diffuser by the gas flow exiting the impeller cause wear in the components of the diffuser to be accelerated by the absence of lubricating oil.
The drive system for accurately positioning the rings relative to one another must, among other things, be rigid to avoid any fretting of components. Because of circumferential loading on the rings there is a propensity for the inner ring to oscillate relative to the outer ring which could cause compressor instability, part wear and could adversely affect efficiency. This causes several problems that need to be overcome. A drive system is needed that is capable of preventing the relative movement between the inner and outer rings. A bearing concept is also needed which would allow for the relative rotation of the two rings and also be capable of withstanding the circumferential and thrust loads while maintaining tight geometric tolerances between the rings. There is also a need to provide a positioning system that includes positive minimum and maximum stops to avoid unnecessary noise and efficiency degradation as well as simple field retrofit. In addition, there is a need for the drive and bearing systems to have a long operating life and be easy to install and adjust properly.